"The Scottish Rugby Union has informed us that a urine test sample provided by Scott MacLeod has shown an irregularity," said Scarlets chief executive Stuart Gallagher.
"As standard procedure, while this irregularity is being investigated further, Scott MacLeod will not be playing or training with the Scarlets until the matter has been resolved.
"The player and the Scarlets management, along with the Scottish Rugby Union, hope to conclude this matter as soon as possible.
"We will assist the player and co-operate fully with the Scottish Rugby Union while the inquiry takes place."
MacLeod denies any wrong doing and has requested to have his B sample to tested in order to prove his innocence.
This is the second time MacLeod has been involved in a drugs-related controversy after he was cleared of failing a previous test involving asthma medication in February of this year.
MacLeod has already missed the Scarlets' weekend defeat against Stade Francais in Paris and could now be in danger of missing out on Scotland's autumn internationals and his club side's last ever game at their Stradey Park home.